1. Field of the Invention
The present invention relates to a device and a method for determining frictional force, more particularly the frictional force (coefficient of friction) between a games ball and a playing surface.
2. Description of the Related Art
Many games require that a ball be propelled by a player to rebound off a surface, as in for example tennis, golf, squash, cricket and so on. Generally, balls for such games are made to rigid technical specifications but surfaces often vary widely in composition and consistency and so the rebound of a ball may vary considerably in terms of its speed and angle. This can introduce significant and often undesirable playing characteristics into such games.
Complex electronic equipment is available for measuring the rebound performance of balls off surfaces consisting of xe2x80x98black boxesxe2x80x99 through which a ball is made to pass before and after obliquely striking a surface. However, such equipment is expensive, and because it is not readily portable and requires mains electricity, it does not lend itself to outdoor use. There is therefore a requirement for a simpler, less expensive measuring device which is portable and easy to use.
In considering the basis for such measurement it is convenient to consider the way in which the ball interacts with a surface by resolving its motion in two mutually perpendicular directions (ie perpendicular and parallel to the surface respectively) and to consider these two components of motion separately. Measurement of the parameters of each component can then be used together to assess the mode of ball rebound from an oblique impact with the surface.
A simple test is available for assessing rebound performance perpendicular to the surface. This consists of measuring the percentage vertical rebound on dropping a ball from a fixed height onto that surface. No such simple test is available for assessing performance parallel to the surface, which requires measurement of the friction generated between the ball and surface during the time they are in contact.
It is an object of this invention to provide a device and a method for determining the coefficient of friction between a games ball and a playing surface.
The object described above is achieved by a device comprising:
a rigid pendulum having first and second ends, mounted by a pivot at its first end to a frame placed on the surface, allowing the pendulum to describe a vertical arc over the surface;
holding means for holding a ball adjacent the second end of the pendulum so as to permit controlled movement of the ball with respect to the pendulum;
adjusting means for adjusting the position of the pivot relative to the surface, to ensure contact of the ball with the surface over a desired length of a lower portion of the arc; and
measuring means for measuring the extent of the arc described by the pendulum following contact of the ball with the surface, so as to allow determination of the coefficient of friction between the ball and the surface.
The object described above is also achieved by the device mentioned above, in which the holding means comprises a holder which is attached to the pendulum by a rotatable shoe arranged such that the plane of the rotation of the shoe relative to the pendulum is perpendicular to the plane of the pendulum arc, and such that the center of the ball can move in a controlled manner in an approximately radial direction with respect to the pendulum pivot, and at a point where the pendulum is vertical, the center of the ball lies on a vertical line passing through that pivot.
The object described above is also achieved by the device mentioned above, wherein the shoe is provided with adjustable stops which control within fixed limits the extent of movement of the ball holder in said radial direction in order to control the length of contact between the ball and the surface.
The object described above is also achieved by the device mentioned above, wherein the shoe is pivoted from a lateral projection of the pendulum, allowing the holder and ball to be disposed underneath the end of the pendulum and thus allowing the ball to move substantially along the axis of the pendulum.
The object described above is also achieved by the device mentioned above, wherein a load is applied to the shoe by means of interchangeable springs so that the force applied by the ball to the surface in a direction normal to the surface may be varied. For example, a tension spring is extended between a point on the shoe and a point part-way along the pendulum.
To determine xe2x80x9cslidingxe2x80x9d friction between the ball and test surface, the ball is firmly fixed in the holder so that it cannot rotate under the frictional forces generated. Alternatively, the ball is mounted in the holder in such a way as to permit either free or restrained rotation of the ball within the holder, to allow determination of xe2x80x9crollingxe2x80x9d friction.
The object described above is also achieved by the device mentioned above, wherein the adjusting means comprises a screw device mounted in the frame at a desired position thereon, thereby allowing adjustment of the height of the pendulum pivot above the surface.
The object described above is also achieved by the device mentioned above, wherein the adjusting means further comprises means for adjusting the position of the ball relative to the pendulum pivot.
The object described above is also achieved by the device mentioned above, wherein the measuring means comprises a scale placed parallel to the plane of the pendulum arc, and a pointer which is rotatable about the pendulum pivot to follow the swing of the pendulum; the pointer is equipped with a holding device, to hold it in position when the pendulum arc reaches its maximum extent following contact of the ball with the surface.
The device of the present invention can be applied to balls and surfaces used in the games of tennis, cricket, hockey and squash rackets among others.
The object described above is also achieved by a method of determining the frictional force between a games ball and a playing surface, comprising the steps of:
retaining a rigid pendulum at one end to a pivot fixed above the surface, allowing the pendulum to swing vertically over the surface;
mounting a ball adjacent the other end of the pendulum so as to permit limited movement of the ball with respect to the pendulum;
adjusting the height of the pivot above the surface, in order to ensure contact of the ball with the surface over a desired length of the lower part of the pendulum swing;
measuring the extent of the pendulum swing following contact of the ball with the surface; and
determining the frictional force between the ball and the surface based on the measured extent of the pendulum swing.
The object described above is also achieved by the method mentioned above, wherein;
in said mounting step, the ball is inserted in a holder attached to a rigid pendulum by a pivoted shoe;
in said adjusting step:
(i) the height of the pendulum pivot above the test surface is adjusted so that the moment of weight of the shoe, holder and ball only is applied to the surface through the ball and a fixed distance is set between the shoe and a fixed upper stop;
(ii) a spring is then fitted to apply a load to the shoe such that it is pressed down against a lower stop when the pendulum is not vertical and such that when the pendulum is vertical the spring becomes extended so that an additional force is applied to the surface through the ball; and
(iii) the lower stop is adjusted to give a ball/surface contact path of known length as the pendulum describes its lower arc; and in said measuring step, the pendulum is raised to a pre-determined height and released so that the ball descends towards, then contacts and slides along the surface as the pendulum describes its arc, and the maximum angle of the pendulum""s swing after contact has been made with the surface is measured by the slave pointer.